Issue 38

A. Eberlein et alii, Frattura ed Integrità Strutturale, 38 (2016) 351-358; DOI: 10.3221/IGF-ESIS.38.45 352 Figure 1 : Crack retardation and acceleration while low-high-low block load sequence; (a) Parameters of a low-high-low block load sequence with constant R-ratio; (b) Schematic a-N-curve after a low-high-low block load sequence according to Richard and Sander [2]. Hereby the definition of characteristic variables can be extracted from Fig. 1 a). A crucial influence on component’s durability has the level of the block loading. As within this experiments also mixed-mode-block loadings are interspersed, the level of the block loading is defined by the block loading ratio R V,block as follows: K R K V,block V,block Bl,max  (1) K V,block is the maximum comparative stress intensity factor during the block loading and K Bl,max is the maximum stress intensity factor of the baseline-level loading. The maximum comparative stress intensity factor can be determined by: K K K K K I,block 2 2 2 V,block I,block II,block III,block 1 5.336 4 2 2        (2) Such a block loading test with a number of block cycles N block causes after a short acceleration phase a higher crack growth rate during the block loading. After that a retardation phase of the crack growth follows, which continues till the crack, if able to propagate, reaches its crack growth rate ( da / dN ) Bl of the baseline-level loading before the block loading (shown in Fig. 1 b)). Apart from changing loading levels also changing in essential loading as well as changing loading directions can appear while product’s operation. Consequently, a variation of the global loading can effect a locally changing of the crack fracture mode e. g. from pure mode I-loading to an in-plane mixed-mode- or a 3D-mixed-mode- loading situation. CTSR- SPECIMEN AND LOADING DEVICE he experimental tests were performed using the CTSR-specimen ( Compact-Tension-Shear-Rotation- specimen) with the corresponding loading device developed by Schirmeisen [3] and can be also found in Eberlein [4]. Fig. 2 illustrates the specimen’s geometry (Fig. 2 a)) and the adjustment of the fracture modes (mode I, mode II and mode III) by the loading angles α and β on the loading device. The corresponding loading device basically consists of two sickles and two inboard so-called turrets, where the specimen is fixed. By varying the loading angle α in the range of 0° till 90° by 15°-steps the mode I-ratio to mode II respectively mode III is regulated. A mounting position of the loading device of α = 0° corresponds with a pure mode I-loading at the crack front of the CTSR-specimen, as in Fig. 2 b) illustrated. Mounting the loading device with the specimen in a position T